Method for measuring and controlling the density of a metallic vapor

ABSTRACT

Method for measuring the density of vapors by the use of electromagnetic radiation such as X-rays or by corpuscular beams such as electrons. A beam of electromagnetic radiation is directed through a metallic vapor to be measured. The density is determined by measuring either scattered electrons, the emission of visible light, or the emission or absorption of X-rays.

MTRDQ lnventor Wilhelm W. B. Schumacher Pittsburgh, Pa. Appl. No. 701,965 Filed Jan. 31, 1968 Patented Oct. 12, 197 l Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

METHOD FOR MEASURING AND CONTROLLING THE DENSITY OF A METALLIC VAPOR 2 Claims, 6 Drawing Figs.

US. Cl 250/43.5 D, 117/933, 118/7, l18/49.1,2l9/12l EB, 250/45, 250/49.5 R, 250/83.3 D, 250/218 Int. Cl G01n 9/24 Field of Search 250/49.5 O, 43.5 R, 43.5 D, 45, 83.3 D, 218; l18/49.1, 7;

References Cited UNITED STATES PATENTS 2,469,206 5/1949 Rich 250/43.5 X 2,952,776 9/1960 Schumacher et al. 250/43.5 D 3,230,366 1/1966 Mielczarek et a1. 250/49.5 (1 3,344,273 9/1967 Ziegler 250/43.5 D 3,373,278 3/1968 Cilyo 250/51.5 X

Primary Examiner.lames W. Lawrence Assistant Examiner-A. L. Birch AttorneysF. H. Henson and R. G. Brodahl 2 l4 MOTOR 10 l i u j 1 l4 2o- MOTOR ,I5 I CONTROL ag 3 "-17? l )1 26 DENSITY A9 la- :7 g CONTROL DEVICE PATENTEUUDT 1219?! 3,512, 59

SHEET 20F 2 8 0 --a- SCATTERING ANGLE IIE/EOE fit w U Q 8 A 3 FIG 6 FIGS.

METHOD FOR MEASURING AND CONTROLLING THE DENSITY OF A METALLIC VAPOR BACKGROUND OF THE INVENTION In coating materials such as sheet steel paper or plastic foil by vapor deposition of aluminum or other material, the resultant thickness of the coating will depend in a reproducible way upon the evaporation rate of the material, or its vapor density. While vapor density depends upon the temperature and area of the vapor source, it is also influenced by such fac tors as oxide layers or other impurities on the surface of the vapor source; and all of these may vary across the width of the strip which is to be coated.

In the usual vapor deposition apparatus, steel strip, paper or plastic foil etc. passes through an evacuated enclosure in which is positioned a vapor source, for instance a heated crucible containing the material to be deposited usually in molten form, but sputtering or sublimation from a solid source is also possible. Due to the fact that the enclosure is evacuated, the metal for instance will readily evaporate from the surface of a molten bath and'be deposited on the surface of moving strip passing overhead. Nonmetals, like sulfur, silicon, quartz or glass may be deposited in a similar manner.

It is, of course, possible to measure the thickness of the coating as the strip emerges from the evacuated enclosure of even while still inside the vacuum enclosure and then adjust the evaporation rate depending upon the measured thickness. The difficulty with this method, however, is that the point of emergence of the strip from the evacuated enclosure or the point of measurement is at a considerable distance from the vapor source. Consequently, considerable time elapses between the occurrence of an off-standard condition and the measurement of that off-standard condition and the generation of an error correction signal which must be relayed back to the vapor deposition apparatus. By this time, however, the condition for which the error signal was generated may have corrected itself or a different off-normal condition may exist. It can be seen, therefore, that this system of control is not satisfactory, especially for a high-speed coating system.

Vapor density can be measured directly by means of a socalled differential ionization gage as it is used in small evaporation units. However, it would be physically difficult to use such gages on large vapor sources since the ion collector has to be not only extremely close to the. vapor but also shielded from it, in order to prevent vapor from condensing on it.

SUMMARY OF THE INVENTION As an overall object, the present invention seeks to provide a method for continuously measuring vapor density directly as the vapor emerges from a molten metal bath or the like.

In accordance with the invention, vapor density may be determined with the use of an electron beam or, alternatively, with the use of x-rays. In the case of an electron beam, a stream of electrons is tired across the vapor, exciting visible and X-ray emission along its path. The electron beam voltage is chosen high enough to obtain a reasonably confined beam in spite of electron scattering in the vapor, a typical beam voltage being in the range of IO kilovolts to 200 kilovolts. The local intensity of the excited radiation from the vapor, whether visible or x-rays, is a direct measure of the local vapor density and can be measured to determine the coating thickness at any given and desired speed of the steel strip.

Alternatively, an electron beam may be fired across the vapor and the vapor density deduced from the intensity of the scattered electrons measured by means of a scintillation crystal and photomultiplier or other electron detectors positioned at an angle with respect to the electron beam.

In the case of vapor density measurement by X-ray absorption, it is preferable to utilize "soft" X-rays having wavelengths in excess of l Angstrom unit. Generation of such X-rays with a conventional X-ray tube is not feasible for the reason that the window in the X-ray tube through which the emitted rays pass will absorb rays of such long lengths. However, due to the fact that the vapor itself is within an evacuated enclosure, the X-rays may also be generated in the same enclosure without the necessity for an exit window.

X-rays and electrons have been used in the past for the purpose of determining the density of gases, such methods being shown, for example, in US. Pat. Nos. 2,952,776 and 2,908,82l,and 3,207,895. However, prior to the present invention, no feasible means was known for utilizing these techniques for the measurement of condensable vapors as contrasted with permanent gases.

The above-and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic diagram of one embodiment of the invention wherein an electron beam is tired across the vapor and the intensity of the scattered electrons measured to determine the density of the vapor;

FIG. 2 is a schematic illustration of an embodiment of the invention wherein an electron beam is fired across the vapor and the resulting X-ray emission measured to determine vapor density;

FIG. 3 is a schematic illustration of another embodiment of the invention again employing an electron beam but wherein the visible emission caused by the electron beam is measured to determine vapor density;

FIG. 4 is a schematic illustration of still another embodiment of the invention wherein vapor density is determined by absorption of soft X-rays;

FIG. 5 is a plot of scatter probability versus scattering angle for an electron beam fired across a source of vapor; and

FIG. 6 is still another embodiment of the invention wherein vapor density is determined by the absorption of soft X-rays from an anode target source as shown in FIG. 4 as well, but having in addition a diffraction crystal for selectively reflecting those X-rays which come from the anode target source but not X-rays which may come from other sources.

With reference now to the drawings, and particularly to FIG. 1, a sheet of strip material 10, such as steel, is shown passing across the top of an evacuated enclosure, generally indicated by the reference numeral 12. As shown, the strip is supported for movement by means of rolls 14 which can be driven by a motor 13 operative with a speed-controlling motor control 15. At the bottom of the evacuated enclosure 12, at one point along its length, is a graphite or the like crucible 16 containing a molten bath 18 of the metal such as aluminum to be evaporated onto the lower surface of the strip 10. A suitable heating device 17 such as a controlled electron beam is provided for maintaining the bath 18 in a molten condition. The bath 18 may, for example, comprise aluminum; and when it is heated within the vacuum, aluminum vapor 20 will rise upwardly and be deposited on the underside of the strip 10.

In orderto measure the density of the vapor, which gives a measure of the thickness of the coating deposited on strip 10 if the moving speed of strip 10 is also known, an electron gun 22 fires an electron beam 24 through the vapor 20 and into a target such as a Faraday cup 26. As the electrons pass through the vapor 20, they will scatter, the amount of scatter being proportional to the density of the vapor. The scattered electrons, in turn, may be detected by means such as a scintillation detector 28 which is also located in the vacuum enclosure 12 and which will produce an output electrical signal proportional to the intensity of the scattered electrons and, hence, proportional to the density of the vapor 20, and ultimately, the thickness of the coating applied to strip 10. The output signal from the detector 28 is supplied to a density control device 19 for the purpose of controlling the energy supplied by the heating device 17 into the molten bath l8 such that a predetermined und desired condition of the vapor 20 is thereby maintuined. It is within the scope of this invention to control the speed of the motor 13 by a signal from the density control device 19, for example, such that the resulting coating upon the strip 10 could be made substantially within a desired thickness range by providing a predetermined movement speed of the strip 10 for a given range of vapor density conditions and slowing down the strip 10 if the vapor condition fell below the latter range and increasing the speed of the strip 10 if the vapor density condition rose above the latter range.

In FIG. 5, scatter probability is plotted versus scattering angle; and it will be seen that the probability varies rapidly from up to about 90. However, from 90 to 180, the probability remains relatively constant. At a scatter angle of 0, the detector 28 would be aligned with the electron beam 24 on the right-hand side of the crucible 16 adjacent the Faraday cup 26. On the other hand, at a scatter angle of 180, it would again be aligned with the electron beam but on the left-hand side of the crucible 16 adjacent the electron gun 22. Since the scatter probability varies rapidly at low scatter angles (i.e., the detector 28 rotated to the right as viewed in FIG. 1), these positions of the detector should be avoided in order to eliminate calibration problems. However, the detector can be position conveniently anywhere between the 90 position shown and the 180 position which would be approached by rotating the detector 28 to the left as viewed in FIG. 1.

If the vapor source is heated by electrons from the heating device 17, which is a quite commonly used method, the heating electrons" are also scattered in the vapor. Provisions have to be made that the electron detector registers only those electrons coming from the gaging beam and not those coming from the heating electrons. To facilitate this selective electron detection, the gaging electron beam is produced with an electron energy substantially higher than the energy of the heating electrons. While the heating electron beam from the heating device 17 usually has an energy in the range of -20 kev., the gaging electron beam from the electron gun 22 should have an energy of from 50-200 kev. Since electrons scattered once through a large angle on the nucleus of a vapor atom do not lose any energy, the scattered electrons from the gaging beam will therefore also possess an energy of 50-200 kev. To detect them, and them only, a foil window can be placed in front of the electron detector which is thick enough to stop all the electrons scattered from the heating electron beam but thin enough to pass the electrons from the gaging beam. For example, an aluminum foil with a weight per unit area of l 'milligram per square centimeter will stop allkev. electrons but pass more than 95percent of the I00 kev. electrons. The electron beam vapor gage can therefore also be used in a system where other electron beams are present, for instance, to heat the vapor source.

With reference now to FIG. 2, elements shown therein which correspond to those of FIG. 1 are identified by like reference numerals. It will be assumed that elements including the crucible l6 and strip 10 are again within an evacuated enclosure, not shown in FIG. 2. As the electron beam 24 passes from the gun 22 to the Faraday cup 26 it will again pass through the vapor 20; and as it does it will emit X-rays, the intensity of the X-rays being proportional to the density of the vapor. These X-rays, in turn, may be detected by a suitable X- ray detector 30 which can take the form of a photographic film, an ionization chamber, proportional counter, parallel plate counter, luminophor or scintillator crystal with photocell or photomultiplier tube. The electrical signal produced by the detecting device 30 will again be proportional to vapor density and coating thickness and can be utilized for controlling the vapor density condition.

In FIG. 3, elements corresponding to those of FIG. 1 are again identified by like reference numerals, it being assumed again that the strip 10 and crucible 16 are within an elongated, evacuated enclosure. As the electron beam 24 passes through the vapor 20, it will emit light, the intensity of the light being proportional to the density of the vapor. This light, in turn, may be focused through lens 32 onto a photosensitive device 34, such as a photocell. Thus, the luminescence will be observed and its intensity determined by means of the photocell 34, thereby obtaining an indication of the density of the vapor The light emission from the vapor (as excited by the electron beam) is usually centered around a few narrow spectral lines. Means can be provided to selectively measure the light of these typical spectral lines only, rejecting all background or stray light (for instance from the hot aluminum bath). Such means may be a spectrometer, a monochromotor, or just an optical filter as shown as element 35 in FIG. 3.

In FIG. 4, an X-ray absorption technique is shown for determining the density of the vapor 20, it again being assumed that all elements are within an evacuated enclosure. A cathode 36 is provided together with a focusing electrode 38. The electrons emitted by the cathode 36 are focused along lines 40 onto an anode or target 42 which emits X-rays which pass through the vapor 20. After passing through the vapor 20, the X-rays pass through a pinhole 44 in a shield 46 and thence onto a fluorescent screen 48. The pinhole produces an image of the X-ray source 42 on screen 48. The intensity of the image line produced on the fluorescent screen 48 is, hence, an indication of the intensity of the X-rays and this intensity, in turn, will be dependent upon the amount of X-radiation absorbed by the vapor 20. The intensity of the visible line produced on the fluorescent screen 48 may, in turn, be converted into a proportional electrical signal by means of any of the well-known photoelectric devices, schematically illustrated by the reference numeral 50. These may include also scanning or picture recording devices like vidicon tubes, etc.

As was mentioned above, the method employed in FIG. 4 is dependent upon the use of a source of X-rays which will be readily absorbed by the vapor. Ordinarily hard X-rays such as those produced by a conventional X-ray tube will not be absorbed by the vapor in sufficient amounts to render the systems practical. Soft X-rays (i.e., those having a wavelength greater than I Angstrom unit) would be absorbed by the exit window of a conventional X-ray tube. In the present invention, however, the use of an exit window is unnecessary since the anode 42 is itself within an evacuated enclosure. In order for the system to work satisfactorily, it is, therefore, necessary to have X-rays which have the maximum absorption for the particular vapor being deposited on the strip 10. In this respect, it is known that the metal from which the anode 42 is formed should have a higher atomic number, on the order of the atomic number of the vaporous material plus 3 to 4 or as close to this number as practical, in order to obtain maximum absorption.

In a system where the vapor source is heated by another electron beam, such as from the heating device 17 of FIG. 1, there are X-rays generated at the surface of the molten metal itself, as well as in the vapor itself. These X-rays have the characteristic wavelengths pertaining to the particular atoms of the vapor. This wavelength is different from the wavelengths of the X-rays coming from our anode target. The X-rays from the anode target can, therefore, be selectively reflected and thereby separated from the X-rays from the vapor as shown in FIG. 6. A diffraction crystal 52 can be used for this purpose, which is as such a well-known device for X- ray reflection and needs no further description here. The X- rays from the anode 42 are reflected from crystal 52 and detected by a phosphorescent screen 54 or any other X-ray detector. The crystal 52 may either be flat, or curved, or curved and ground (cylindric or spherical), in order to achieve a focusing action. Systems of this kind are well known in the art of X-ray optics.

A special feature of all the gaging systems so far described has not yet been stressed. With reference to FIG. 1 we see that the electron beam traverses the vapor zone at right angles to the long axis of the vapor source, in other words it traverses the vapor on the shortest possible path. If the vapor density in the middle of the vapor zone is higher than near the edges, which is to be expected, then more electrons will be scattered from the middle region of the vapor zone than from the edges. If, now, detector 28 is made to see only the middle section of the vapor zone, for instance by placing a set of suitable apertures in front of the detector 28, the signal received by the detector 28 will correspond to the vapor density in the middle of the vapor zone only. Similarly the same detector could be turned around to look at the edge of the vapor zone, or a second, third and fourth detector with a narrow field of view could be used to observe other parts of the vapor zone than the middle region. Obviously this permits observation of the local value of the vapor density.

If the system consisting of electron gun 22, scatter detector 28, and electron beam 26 in FIG. 1 would be rotated through 90 the electron beam would pass through the vapor zone along the long axis of the vapor source. As has been explained out of the previous paragraph, it would then be possible to look at the electron scatter intensity from any localized area along the long axis of the vapor source, and measure the local vapor density, say, on each end of the vapor source. The relative electron scatter intensity at each end of the vapor source will be a measure for the relative vapor density at each end, and the measuring accuracy will be particularly high because it is one and the same primary electron beam which passes through those regions and which has, of course, the same intensity. (Only about one out electrons is being scattered and this does not change the intensity of the primary beam to a noticeable degree while passing through even the very long vapor zone parallel to the long axis of the vapor source.)

The same kind of localized density measurement can of course be made with the system shown in FIG. 2 and FIG. 3.

The fact that the local vapor density can be measured is of the greatest practical importance insofar as for a vapor source which is rather long, for instance as shown as element 18 in FIG. 1 or 2. The heating at both ends may be uneven producing an uneven vapor density at both ends, or oxide or slag layers may have accumulated on part of the surface of the liquid metal, impeding the vaporization, and in such case the coating thickness produced on strip 10 would be uneven. In using just one vapor-gaging beam of electrons we can detect and subsequently correct such unwanted conditions.

If we are using an X-ray beam for gaging as shown in FIG. 4 we also detect local variations in the vapor density in view of the fact that we have a narrow long X-ray source which is as wide as the vapor source is long. Since X-ray absorption is affected over the whole path length of the X-rays through the vapor, and this path is rectangular or nearly rectangular to the long axis of the vapor source 20 in FIG. 4 the measured value of vapor density is an integral value over the density along the path of the X-ray beam. As long as this path is parallel or nearly parallel to the direction of movement of strip 10 this integrated vapor density over the X-ray path is still a measure for the coating thickness on strip 10 because each part of the vapor zone parallel to the direction of travel contributes to the coating in this area of the strip 10.

The integrating feature of the X-ray gaging system can be regained or added to the electron beam system if the electron beam is not kept stationary but, for instance, deflected through the whole width of the vapor zone, thereby producing what one may call a line-scan of the whole three dimensional vapor area. The signal from the scatter detector, or X-ray detector, or light detector will then correspond to the vapor density at the momentary position of the electron beam. By electronic means these instantaneous signals can be displayed to give a three-dimensional picture of the vapor density distribution on a recorder chart or an oscilloscope screen; such data display systems are well known in the art and need no further description here.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes may be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In the method for controlling the density of a metallic vapor, the steps of:

producing such metalic vapor within an evacuated enclosure by evaporation from a metallic bath;

producing and directing an electron beam of high energy throughsaid metallic vapor heating said metallic bath with electrons having an electron energy lower than that of said high-energy electron beam;

passing the electrons which are scattered by said metallic vapor through a filter to arrest passage of the scattered electrons of said lower energy;

detecting and measuring the intensity of the unarrested scattered electrons of high energy to thereby determine said density; and

controlling the production of said metallic vapor to control said density.

2. The method of claim 1 wherein said electrons of lower energy have an electron energy in a range of about 10 to 20 kev. and said electrons of higher energy have electron energy in the range ofabout 50 to 200 kev. 

2. The method of claim 1 wherein said electrons of lower energy have an electron energy in a range of about 10 to 20 kev. and said electrons of higher energy have electron energy in the range of about 50 to 200 kev. 